Silica sol process



Patented Mar. 10, 1953 UNITED STATE S PATENT OFFICE SILICA SOL PROCESSIRalph K.

ton, Del., assignors Delaware a corporation of No Drawing. ApplicationNovember 7, 1951, Serial No. 255,335

8 Claims.

This invention relates to the preparation of silica sols by processes inwhich the alkali in an alkali metal silicate solution is neutralizedwith a cation-exchanger in the hydrogen form while maintaining a pHabove 8. The invention in one specific embodiment is more particularlydirected to processes in which the cation-exchange is efiected with analkali metal concentration of less than 0.35 normal, at a temperatureabove 60 C., and over a period of time not less than about one-halfhour, whereby stable silica sols made up of particles of relativelylarge particle size are produced.

This application is a continuation-in-part of our copending applicationSerial No. 128,243, filed November 18, 1949, now abandoned.

Silica sols have been made by neutralizing the alkali of an alkali metalsilicate solution with a cation-exchanger. Bird Patent 2,244,325 showssuch processes.

In the process of the Bird patent and in similar processes using thehydrogen form of a cationexchanger, a solution of an alkali metalsilicate is brought into contact with the cation-exchanger at a pH whichis initially quite high.

After the cation-exchange has been effected, the pH ends up at a lowfigure, usual y in the range of pH 3 to 5 in commercial operations. Insuch processes alkali is ordinarily added after the exchange tostabilize the sols. Sols cannot be made by such processes which have anSiOz content above 3 or 4 per cent, as they leave the cationexchanger,because they will gel in contact with the cation-exchanger at higherconcentrations.

. Alkali can 'be added to the sols so made and then they can beconcentrated. These sols are fairly stable at $102 contents up to aboutper cent.

Silica sols prepared by neutralization of an alkali metal silicatesolution with a cation-exchanger can be markedly improved by causing agrowth of the silica particles to form dense, uniform, discreteparticles. This process is described in an application of Max F.Bechtold and Omar E. Snyder, Serial No. 65,536, filed Decem ber 15,1948, now Patent No. 2,574,902. The processes of Bechtold and Snyder arecharacterized by heating of a portion of sol and thereafter adding atleast five times as much silica as was originally present by theaddition of further quantities of sol.

The present invention permits the direct production of sols of higherSiOz content than those which can be prepared by prior art processesusingthe hydrogen form of a cation-exchanger.

By using a process of the invention an alkali metal silicate solutioncan be treated with a cation-exchanger in the hydrogen form to produce asilica sol directly which contains a much higher SiOz concentration thancan be obtained by prior processes using the hydrogen form of acation-exchange resin.

The present invention also permits the direct production of sols by the,treatment of alkali metal silicates with cation-exchangers in thehydrogen form under such conditions of temperature, alkali metal ionnormality, pH, and time as results in the direct formation of sols ofthe type shown in Bechtold and Snyder. These sols are composed ofparticles which are quite dense and which have a particle size fromabout 10 to millimicrons. The sols can be concentrated to, say, 30 percent S102 and even at such concentrations are quite stable for extendedperiods of time.

Any soluble alkali metal silicate may be used. Sodium silicate willusually be described throughout the specification since this is thecheapest of the soluble silicates, but it will be understood thatpotassium silicate is equivalent. The silicate used may have any molratio of SiOzZNazO from 1:1, that is metasilicate, up to 4:1, or saymore practically, 3.9:1.

According to the invention the alkali of an alkali metal silicatesolution is neutralized with the hydrogen form of a cation-exchanger.Cation-exchangers in the hydrogen form suitable for neutralizing alkalimetal silicates are described in the Bird Patent 2,244,325 and in theHurd Patent 2,431,481.

Any insoluble cation-exchanger in the hydrogen form may be used inprocesses of the invention and there may be used, for instance,sulf-onated carbonaceous exchangers, sulfonated or sulfited insolublephenol-formaldehyde resins or acid-treated humic material, or othersimilar exchangers. Sulfonated coal, lignin, peat, or other insolublesulfonated humic organic material may be used.

Even more preferable are the insoluble resins made from phenols, such asthose made from phenol itself, diphenylol sulfone, catechol, ornaturally occurring phenols, as found, for example, in quebracho, and analdehyde, particularly formaldehyde, which are modified by theintroduction of sulfonic groups either in the ring or on methylenegroups.

Cation-exchangers which are stable in their hydrogen forms are availablecommercially under such trade names as Amberlite," Ionex, Zeokarb,Nalcite, Ionac, etc.

It is, of course, preferred that the resins selected be comparativelystable at the temperature and alkalinity of the processes of theinvention. While the unstable cation-exchangers may be used a few times,they cannot under practical conditions of operation be re-usedcontinuously.

One of the suitable cation-exchange resins for use according to thepresent invention is an aromatic hydrocarbon polymer containing nuclearsulfonic acid groups which is designated Dowex- 50 and of the generaltype described DAlelio 2,366,007 and which is fully described as to itslinking being conducted, for instance, with divinyl benzene. In thepublication cited G. E. Boyd, on page 318, describes a carboxylic typecation-exchanger prepared by polymerizing methacrylic acid with aboutten per cent its weight of divinyl benzene using a peroxide catalyst.This general type of product is commercially available as AmberliteERG-5.0.

The exchanger is generally prepared in a granular form which is readilyleached free of soluble acids or salts. If the exchanger .is exhaustedbyuse it may readily-be converted to the acid form by washing with asolution of an acid such as hydrochloric, sulfuric, sulfamic, andcarboxylic, such as formic. It will be obvious that the regeneration"will be much more efficient when the regenerating acid is a strongeracid than the acid groups on theresin.

Now in the processes of the present invention a solution of an alkalimetalsilicate as above described is brought into contact with theacidform of a cation-exchanger. The alkali of the .sili- ,cate solutionis neutralized by the cation-ex- ;changer, whilemaintaining-a pH above8. When ,itxis said that the alkali metal silicate solution isneutralized, it will be understood that by this it is meant that thealkali metal ion is exchanged and the pH is lowered to a figure belowthat of the original solution. An alkali metal silicate solutionordinarily will have a pH some- -what-above 11.

Throughout the reaction of the alkali metal silicate solution with thehydrogen form a cation-exchanger the pH is maintained above 8 as hasbeen noted. It is preferred that the pH throughout be maintained at afigure no higher I than 10.5.

The extent of the neutralization, or of the removal of alkali metalionfrom the alkali metal silicate solution, can most easily be expressed interms of the SiOzzMzO ratio in the sol produced.

It has been noted earlier that the silicate solution used at thebeginning has a ratio of SiOzzMzO no higher than about 4:1. This ,is amolar ratio in the case of alkali metal silicates generally, and is alsoapproximately .a Weight ratio in the case of .sodium silicate.

.Any SiOzZMzO molar ratio between 10:1 and 150:1 can be attained. Theextent of neutralization .of the alkali in the alkali metal silicatesolution can be varied within rather wide limits. Any SiOzIMzO molar,ratioabove about 10,:1 can be obtained. vIn preferred processesof theinvention the siozzlvno ratio willbe maintained be- 'tween about 10:1and 150:1. vIt ismore specifically preferred to maintain a ratiofromaboutBO:1'tol50z1.

in a range In the expression of ratio as SiOztMzO the letter'fM is usedas'is customary torepresent the alkali metal, such as sodium orpotassium.

If higher ratios than 150:1 aremaintained .exceed about 75,000 squaremeters. millimicron particles, a silica concentration of about 12-713,grams of SiOz per .100 milliliters is about the upper practicaloperating limit, since during the processes of the invention or arereached at the end of the process, it will be found desirable to have aminimum of impurities in the system if a sol of any stability is to beproduced. Impurities such as sulfate, chloride, and the like shouldeither be excluded by the use of chemicals which are comparatively freeof these materials, or they should be removed.

While operating a process of the invention and while maintaining the pHbetween 8 and 10.5, it is important that the alkali metal ionconcentration be kept at less than 0.35 normal and preferably less than0.2 normal. The alkali metal ion concentration can be kept to a lowfigure by increasing the amount of cation-exchanger or by .using acomparatively smaller amount ofalkali ,metal silicate solution comparedto the volume of the aqueous system to which it is added. vIn otherwords, the alkali metal silicate shouldnot be brought into contact withthe cation-ex- .changer at so great a rate that the concentrationofalkali metal ion rises above the f gure indicated. The cation-exchangershould be allowed to-keep up with the addition.

Thetotal amount of Water which isin contact with the cation-exchangershould be sufiicient to permit good contact between the exchanger andthe alkali .metal silicate. Thus, the aqueous system together with theresin should not .be so thick as to preventgood agitation and effective:mixing of all partsof the system. To this end,

the silica content of the sol should also be limited. .If too muchsilica is present in the aqueous phase, some gelling or aggregation will.occur. The resin may even become coated with silica.

II'he upllerlimitof SiOz concentration will vary with theparticle sizeof the SiOzin the sol. With very small particles, .the SiOzconcentration. may

not exceed, say about 7 per cent. On the vcontrary, .for very. largeparticles. which are near the upper limit ofthe colloidal range, theS102 .con-

tent may be much higher, and may be as high, for example, as 40 percent. If the sols areiree of impurities .they can be more concentrated.

The tendency of the sol to geldepends upon, among other factors, thetotal surface area of .the silica particles present in a given volume ofsolution. Thus, under the conditions of this process, ,it is preferredthat .the total surface of the silica particles in one liter of the solnot Thus, for 5 silica particles having an average diameter of about5millimicrons have a specific surfacearea of the order of 600 m-. /g.For particles having an average diameter of 10 millimicrons, thiscorresponds to .an .upper operating limit of about 25 grams of ,SiOz per,100 milliliters of solution.

It will be evident that in processes such as some of those hereinafterdescribed .in which the particles are caused tobecome larger during thecourse of the process, the concentration may .beincreased during thelatter stages as the particle size increases.

Processes of theinvention can be conducted in very dilute solutions suchas 1 per cent 51102, but there islittle advantage. The sols produced bycontact with the cation-exchanger will ,preierably have an SlO2 contentof 3 per cent orinore and, in general, since it is desired to makeconcentrated sols, the SiOz content will be as high as is feasible. Forexample, and as will be shown ticles and having greatest stability areto be produced. Again, it is greatly preferred to use a temperature inexcess of about 90 C., as will be hereinafter discussed in connectionwith a further specific embodiment of the invention.

The time during which the neutralization is efiected and the processconditions are maintained can be widely varied depending upon the typeof product desired. As will be observed hereinafter, a comparativelylong period of time is required to produce products of greateststability and largest particle size. Products of a very useful kind,however, may be made by conducting the process over as short a time aseven a few minutes. If the process conditions above outlined arecarefully met, then the addition can be effected as rapidly as ispractical. Generally the time will be ten or fifteen minutes, or evenlonger. For the production of sols having a particle size greater thanabout ten millimicrons a longer time should be used, as will behereinafter set forth.

The manner of effecting contact between an alkali metal silicate and acation-exchanger is not at all like that heretofore used in which thesolution is passed slowly through a bed of the cation-exchanger. In sucha prior process the pH and the ratio in the solution varies from pointto point in the bed of ion exchanger.

In processes of the invention the alkali metal silicate solution shouldbe brought into intimate contact with all of the cation-exchanger tomaintain a uniform and homogeneous condition throughout the system as topH and ratio. While this can be effected by passing an alkali metalsilicate solution very rapidly through a bed of ex changer and thencontinuously re-cycling the liquid, it is preferred to use a simplermethod in which the cation-exchanger and alkali metal silicate solutionare agitated together in any suitable manner, the resin thus ordinarilybeing in the form of a suspension or slurry.

A preferred method of effecting contact between the alkali metalsilicate solution and the hydrogen form of a cation-exchanger is to addcation-exchanger and alkali metal silicate solution simultaneously oralternately to a mixing For example, the two components may troughprovided with zone. both be added to a pipe or means for agitation.

Again the two solutions may be run into a comparatively small receptaclein which a body of the mixture is maintained while being agitated, andthen the mixture may be continuously withdrawn from the mixing zone.

It will also be apparent that the two reactants can be added to a largerbody of liquid'and permitted to accumulate until a batch of the desiredsize is reached. Again, a comparatively large body of a suspensionresulting from a previous reaction of silicate and cation-exchanger canbe withdrawn through a pipe and re-cycled and the two reactants added tothis stream, which is then discharged into the main body of liquid.

Alternatively the hydrogen form of a hydrogen exchange resin may besuspended in water and a sodium silicate or an alkali metal silicatesolution may then be added at such a rate as to maintain a pH within therange as already described. This is particularly effective where thecationexchanger withdraws sodium ion from the solution sufiicientlyslowly that pH control is no great problem. This is especially the casewith the weakly acidic cation-exchangers such as the carboxylic ionexchange resins. During the process further quantities of resin may beadded to the aqueous system as needed.

Another method of operation is to start with a dilute solution of sodiumsilicate and to add thereto a cation-exchanger in hydrogen form. Againthe rate will be such as to maintain the pH at the desired figure.Further quantities of silicate can be added. It should be noted that inthis case the starting sodium silicate solution should be of suchdilution that the sodium ion concentration does not exceed 0.35 normalafter the SiOzZNazO ratio reaches about 10:1.

In other words, the sodium silicate solution thus used should preferablynot contain more than about 10 per cent SiO2 by weight.

It will be noted though, that it is not in general preferred to add theexchanger to sodium silicate. It is easier to maintain the conditionsabove described when the silicate is added to a slurry of the exchangeror when the exchanger is added simultaneously with the silicate.

It will be evident that there are numerous other ways commonly used foreffecting complete con tact of a solid material with a liquid, and thatany of these may be used.

After the preparation of sols according to processes of the inventionand under the conditions outlined has been completed, one may thereafterseparate the exchanger from the sols produced. This may be done, forinstance, by filtering, decanting, or centrifuging.

After the sols have been produced as described and after, or before,separation of the cationexchanger the pH may be lowered by the additionof further quantities of the hydrogen form of a cation-exchanger. Or, ifthe cation-exchanger is not fully exhausted, the mixture may be stirreduntil the pH drops below 8 to whatever value is desired. The sols caneven be stabilized temporarily on the acid side in accordance with knownpractices.

The sols produced according to processes of the invention may be usedfor a variety of purposes as have silica sols of the art. The solsproduced which have particle sizes below about 10 millimicrons will befound valuable as nuclei in the preparation of stable sols according toprocesses such as those of Bechtold and Snyder described above. Sols oflarger particles can be concentrated, as will be shown hereinafter. Solsproduced according to the processes as described may be used for makinggels or silica powders.

It will be noted that the limitation on sodium ion normality is to beobserved only during the neutralization and preparation of sols byprocesses according to the invention. After the sol has been producedthe sodium ion normality can be raised if it is desired to cause gellingor to form fine, precipitated silica products. This can be done by theaddition of sodium silicate, sodium sulfate, or another sodium salt to afigure above 0.35 normal, and the silica will be precipitated from thesolution in the form of a fine pulverulent product. The precipitationcan also be facilitated by addition of water-miscible organic materialswhich lower the dielectric constant of the medium. Such water-miscibleorganic solvents as .methyl, ethyl, :propyl, :or tertiary butylalcohols.

.iketones .such 'as ;methylethyl vketone, or acetone, "acetamide, ethersof ethyleneglycol, and thellike v-may baused. To ,efiect theprecipitatiomemploy- :ing the solvents,-the salt concentration need not.be raised above 20.35 normal, for even at this (figure orevensomewhat.below, precipitation of "the silica can be efiected. The pH during the:.precipitation should be-above 6.

It will be understood that the foregoing de- 'scription of processconditions is generally applicable to processes ,for producing solshaving .gmarticlesfrom a very small size up to the upper limit ofcolloidaldimensions. If it is specifically desired to prepare solsdirectly which have a parxti'cletsizeaboveqabout l0. millimicrons of thechar- "acter of those shown in theBechtold and Snyder -=applicationabove cited, the process conditions :now "to be described :may be used.

The pH during theprocess will 'be controlled asg'generally describedabove and will beheld to eafigureabove pH 8,-and preferablybetween Band10.5. The alkali metal ion normality will be as previously described.The ratio similarlywill be -within a preferred range from. about 60:1 to150 :1.

:During the course of the neutralization as vz-above described using thehydrogen form of a :cation-exchangerto produce products having aparticle size above about 10 millimicrons, the temperature of the systemmust be maintained aboveabout 60 0., and it is greatly preferred to usea temperature in excess of 90 C. The

neutralization may actually .be eiIected at boil- ;ing temperatures orat temperatures in excess "of boiling temperatures as obtained byincreasv.ingthepressure. ,Asa practical matter the temperatures willusually .not exceed about 100 10.,

:and certainly'will not exceed about 150 C. because of the difiicultiesof constructing suitable equipmentand becauseof the efiect upon cation-.exchangerss PI-he period 'of time over which the neutralization-iseffected, -i. e., the total time required for the addition of the-resinto the silicate solution,

:under the conditions described is a function of the temperature. Thetime of neutralization ..must .be at least (2 x45 minutes, Tbeing thetemperature in degrees C. These times represent minimums, andthe minimumtime for neutralizing at representative temperatures will be seen fromthe following tabula- The time thus referred to is applicable to thecase where the hydrogen form of the cation- -exchanger is added at -,-auniform rate to a solution of an alkali metal silicate containing, forinstance-6 per cent S102. The time of neutralizationis measured from thepoint in the neutraliza- -.ti.o.n where the SiO22M2O mol ratio is 6:1until the desired end-point, where essentially all of athe alkalimetal-ion has been removedisreached.

.The time can be much longer-and maycontinue for a period of hourswithout dilficulty. ,The

.preferred times forpractical operation will ordinarily'be 2 or 3 timesas. long as .thestated mini- .mums. .Also, it will be observed thatwhile the solids) :minimum time .is strictly that required after the.SiOziMzO ratio .rises above 16:1, as -a practical matter the total timewill be used. .There is no practical necessity for determining the molratio in commercial operation and the neutralization will be efiected ata .rather regular rate over a period at least equal to the minimumsgivenand preferablysomewhat over the minimums.

.In carrying vout a process as just above described a solution of analkali metal silicate is first prepared and to this is added the cation-,exchanger in hydrogen form. The addition will .beat a regular rate and,if desired, further quan-- titles :of silicate and resin can be added tothe system to grow particles of stilllarger size.

The build-up ofthe particles in the 501 may from this point on becarried out very much as in the Bechtold and Snyder application abovementioned except that the active silica is formed in .situ by theneutralization of the alkali metal silicate with the hydrogen form of acation-exchanger. The :amount of build-up may be, as in BechtoldandSnyder, as much as 5:1, oreven 1nore,'to obtain sols of great stabilityand particles of large size.

In order that the invention may be better understood the followingexamples are given in addition to those already described:

Example 1 In this example, the reaction is carried out by thesimultaneous addition of silicate and resin to a reaction vessel whilecontinuously withdrawing the silica sol thus formed.

The reaction is carried out in a four liter stain.-

' less steel beaker with an overflow pipe, which maintains the .contentsliters; at the outset the beaker is filled with water. A solution ,ofsodium silicate (3.2-5 SiOzzNazO, 10 per rent $102) .is fed into thebeaker at arate of '25,ml./min. Simultaneously wet, drained fAmberliteIRS-50 (hydrogen 'form) is .fed into the beaker at such a rate as tomaintain the pI-Iat 9:03. The temperature is maintained at about 60 C.The system is vigorously stirred during silicate addition. Theoverflowis dropped onto afilter and filtered immediately, thefirsttwolitersof filtrate being discarded. .The addition of resin and silicateis continued .ior .several hours.

The product .is a silica sol having a pH of about 9, an SiO2:NazO .ratioofabout :1, and containingabout ten per cent S102.

Example .2

This is an example of the preparation of a silica sol containing about 8per cent SiOz and having particles which are about 7 miilimicrons indiameter. In this example, silica deposition on the resin is very low.

A slurry of 850 gm. of'wet of the beaker at two drained (50 per centAmber-lite IRC-50 resin in the hydrogen form, and 1000 ml. of water washeated C. To this slurry was" added 1000 ml. (1196 gm.) of a sodiumsilicate solution (20 g. SiOzzlOO ml. and SiO-z:Na2O=3.25) according tothe following schedule.

Time (Mm ii l ic ate T 00.

This slurry was filtred, using a filter cloth on a Buchner and applyingvacuum for about 15 seconds, yielding about 1400 g. of filtratecontaining about 12 per cent SiOz. The resin on the filter was washedwith 800 ml. of water, the filtrate containing about 3 per cent 8102.The two filtrates were mixed. The SiOzzNazO ratio in this sol was about100:1.

The pI-I of this mix was lowered to 7.0 by adding 175 g. of wet resinand stirring for about 15 minutes. The slurry was then filtered and theresin was washed with 100 ml. of water.

The product was a silica sol containing 8.2 per cent SiOz and havingparticles about '7 millimicrons in diameter, which corresponds to adried silica having a surface area of 388 mfi/g.

A sample of the resin used in this experiment was washed thoroughly withdistilled water and dried at 125 C. The dried resin contained about 0.2per cent S102.

Example 3 This example is similar to Example 2, except that the reactionwas carried out at 70 (3., and the surface area of the silica in thefinal product was 452 m. /g.

Example 4 This example is similar to Example 2, except that the reactionwas carried out at 90 C., and the silica in the final product had asurface area of 388 mP/g.

Example 5 This example is similar to Example 2, except that the rate ofaddition of the silicate was about u 1% times as fast as in Example 2,and the silica in the final product had a surface area of 441 /g. 7Example 6 Example 7 This example illustrates the preparation of a silicasol by a process in which a sulfonic acid resin was fed simultaneouslywith a sodium silicate solution into a heel of water.

The reaction was carried out in a one liter beaker. During the reactionthe slurry was vigorously stirred. Two hundred milliliters of water wasadded to the beaker and this was heated to a temperature of 90 C.Thereupon, simultaneously but separately, samples of resin and sodiumsilicate were added to the solution. The resin was Nalcite HCR, in thehydrogen form, which was drained dry, the resin containing about 50 percent solids when dried at a temperature of 125 C. About 28 grams ofresin was added every four minutes. The sodium silicate solution wasprepared by diluting 1410 grams of F" grade sodium silicate (having anSiOz content of 28.4 per cent and an SiOmNazO mol ratio of 3.25) to atotal volume of two liters. This sodium silicate solution was fed intothe reaction vessel at a rate of 10 milliliters per minute. During thisprocess, the pH was held in the region or" about 9.5 and the temperaturein the range between 90 and 95 C. The additions were continued over aperiod of about 32 minutes, during which time 320 milliliters of sodiumsilicate solution were added. After the deionization, the slurry wasfiltered, the filtrate had a pH of 9.25. The filtrate 10. was analyzedand found to contain 13.5 per cent SiOz.

In order to determine the size of the silica particles in the sol, thesilica was recovered from the solution by dropping the pH of thesolution to 7 with ion exchange resin, warming the solution until itgelled, adding n-propanol to the gel and removing the water byazeotropic distillation. The dry silica powder thus obtained had asurface area of 301 m. /g.

A sample of the resin used in this preparation was analyzed in order tosee how much silica had deposited on the resin during the process. Itcontained 0.24 per cent S102, based on the dried resin.

Example 8 This is an example of the preparation of a silica solcontaining very small silica particles by the addition of an ionexchange resin batchwise to a solution of sodium silicate.

The solution of sodium silicate containing 10 grams of $102 per ml., andhaving an SiOz/NazO mol ratio of 3.25, was heated to 50 C. To one literof this solution there was added, with vigorous agitation, 25 grams ofAmberlite IRC-50 (wet, drained, regenerated resin in the hydrogen form,containing about 50 per cent solids when dried at C.) every two minutesfor a period of 15 minutes.

This mixture was filtered, the filtrate having a pH of 10.3. Thissolution was then cooled to about 15 C., and further deionized by theaddition 01: fresh Amberlite lRC-50 resin until the pH dropped to 6.This slurry was then filtered, the silica sol was allowed to stand untilit gelled.

When the silica in this gel was recovered by removing the water byazeotropic distillation from n-propanol. the gel was found to have asurface area of 680 m. /g., corresponding to silica ptarticles which areabout 4 millimicrons in diame er.

Example 9 Five hundred ml. of a sodium silicate solution containing 10grams of SiOz per 100 ml. was placed in a beaker with a stirrer andheated to 80 C. To this solution, grams of Amberlite IRC-50 resin in thewet, drained state (containing 50 per cent solids) was added. Thissolution was stirred vigorously, and after 1 minute, 250 ml. of a secondsolution of sodium silicate containing about 150 grams of SiOz perliter,

Example '1 0 In this example Amberlite IRC-50, a polycarboxylic resin inthe hydrogen form was placed in a column. Water was circulated at highvelocity (20 bed displacements per minute) through this column. To thiswater there'was added a sodium silicate solution, containing 28 per centS102, and having an SiOz/NazO ratio of 3.25. In the experiment, 500.mls. of wet, drained resin was used, and 600 mls. of water. To this, 400grams of sodium silicate were added over a period of 30 minutes. Thetemperature during the reaction was maintained between 30 and 40 C. Thesilica sol thus obtained was separated from the resin by draining. Thissilica sol had a pH of 10.0, and an analysis of about 12 per cent S102.

ssists- Example 11 A solution of sodium silicate having an Si02rNa2Oweight'ratio of 3.25 and an S102 contentof 4 per cent by Weight wasplaced in a reaction vessel provided with means for agitation andheating. The solution was heated to 95 C. and Dowex 50-G was added in anacid form having been regenerated with sulfuric acid and being in awashed, wet, drained condition. The exchanger was slowly added to thesolution'of silicateover a periodof two hours. The totalamount of resinadded was sufiicient to lowerthe pH of the'solution finally to about9.5. During the period of addition and heating there was agitation ofthe system.

The result was a ilica $01 which was separated by decantation fromtheresin. The silica sol had an SiOz content-slightly lower than theoriginal silicate because of the water introduced on the resin The solwas concentrated to per cent SiOz by weight by the direct evaporation ofwater. The sol is stable over a period of months of standing at normaltemperatures and is suitable for treatment of textiles, inclusion inwaxes,- and so forth;

Example-12- A sei reparea as in Example 11 anacomaming about 4 per centSiO2 was placed in a vessel and heated to- 95 C. A resin'of the typeused in-Examp'le 11 was slowly added simultaneously with a'sodiu'nisilicate solution such as that of Example 11; excepting that itcontained 12 per cent S102 by weight. The proportions of the resin andsilicate were such as to maintain a pH in the neighborhood of 9 to 10.The solution was, of course, agitated at all times and the rate ofaddition of the-two materials was such that one part -by weight ofsilica in the incoming solution wasadded for one part of the originalsilica in the starting solution per hour. This was con tinued for aperiod of six hours so that 6 parts of silica in the form of sodiumsilicate solution was added for each part of silica present in theoriginal sol.

During the run thesolution increased in concentration andin turbidityand the particle size of the fine particles in the sol was increased.The final solhad an SiOz content by weight of about 11 per cent. ThefinalpI-I of the sol as measured at ,30 C. on a cool portion, in theabsence of.

resin, was 8.5,this value being obtained by minor adjustments of resinand sodium silicate solution at the nd.

The sol was separated from resin, heated for a period of a half an hourat 95 0., and then concentrated by direct evaporation to a concentrationof 30 per cent SiOz.

We; claim: V

1. In a process for the production of a silica sol, the steps comprisingneutralizing the alkali in an alkali metal silicate solution with acationexchanger in the hydrogen form while maintainmgapnaboves. it

a process for the production of a silica sol, the steps comprisingneutralizing the alkali in an alkali metal silicate solution with acationexohang'er in the hydrogen form while maintaining a pH above 8,and an alkali metal ion concentration of less than 0.35 normal.

3. In a process for the production of a silica sol, the steps comprisingadding an alkali metal silicate solution to a mixing zone, maintaining aprisaid zone between a and 10. 5 and an alkali metal ion concentrationor less than" 0.35 normal, and withdrawing the resulting $01 fromthemixing'zone.

i. In a process for the productionof asilica; sol, the steps comprisingadding an alkali metal silicate solution and a cation exchanger in thehydrogen form to a mixing zone wherein the pH is maintained between 8and 10.5, and continuously withdrawing the resulting sol from" saidzone.

5 .In a process for the production of a silica sol, the steps comprisingneutralizing the alkali in analkali metal silicate solution by bringingsaid silicate and a cation -exchanger together while maintaining a pHbetween 8 and 10.5, and? an alkali metal ion concentration of less than0.35 normal, the neutralization being effected at 'a temperature between60 C. and 150 C. and over a period of time of at least minutes, T beingthe temperaturein degrees C.

6. In a process for the production of a silicaso'l, the steps comprisingadding sodium silicate solution having an SiQzZNazO ratio below 4:1 to acation-exchanger to raise the ratio to between 10:; and-150:1, while thechange in ratio is being efiected maintaining apH above 8, asc

dium ion concentration of less than 0.35 nor-'- mal, and a temperatureabove 60 C., the time of raising the ratio being minutes, T being thetemperature in degrees C.

7. In a process for the production of a silica sol, the stepscomprising'addin'g a sodium silicate solution having an SiO2-ZNa2O ratiobelow 4:1

and a cation-exchanger in the hydrogen form to s'ol, the stepscomprising neutralizing the-alkali in an alkali metal silicate solutionto an.

SiOzIMzO ratio from 1051 to" 150:1 by bringing said silicate and acation excha'ngei together while maintaining a pH above-a and an alkalimetal ion concentration oi les's'than 0.35 normal, the neutralizationbeing effected at a temperature between 60 and 150" C.- and over aperiod of time or at least 7 'i' (2 x45) minutes, T being thetemperaturein degrees C.

RA ii E FREDERICK J. WOL'I'ER.

' REFERENGES GITED The fonowiiig references are or record in the file ofthis patent:

UNITED STATES PATENTS

1. IN A PROCESS FOR THE PRODUCTION OF A SILICA SOL, THE STEPS COMPRISINGNEUTRALIZING THE ALKALI IN AN ALKALI METAL SILICATE SOLUTION WITH ACATIONEXCHANGER IN THE HYDROGEN FORM WHILE MAINTAINING A PH ABOVE 8.